Adsorption process



Aug. 9, 1966 Filed March 19, 1962 FIGUREIII 2 Sheets-Sheet 2 (I) LLI 2 S g 2% 3 .n, 5 22 a a: o (I) O Z 9 Lu 22 [1.105 m- 22 '3 2 INVENTORS:

8 HARRY o. EVANS RICHARD J. SCHOOFS THEIR ATTORNEY United States Patent 3,265,755 ABSORPTION PROCESS Harry D. Evans, Oakland, and Richard J. Schoofs, La-

fayette, Calif-2, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Mar. 19, 1962, Ser. No. 180,459 9 Claims. (Cl. 260-674) The present invention relates to an improvement in the art of isomerizing parafiinic fractions to improve octane quality. More particularly, the invention is directed to a novel adsorption process for preparing an isomerization feed stream to improve the efficiency of the isomerization process. I

With the modern trend toward the use of high compression ratios in automotive engines there has been an increasing demand for motor fuels of higher octane rating. One of the processes employed for improving the corresponding isoparaffins.

When the light parafiin fraction that is being isomerized contains appreciable amounts of aromatic hydrocarbons such as benzene, the efficiency of the isomerization reaction is seriously impaired. The presence of benzene in the feed in an isomerization reaction in quantities greater than about 0.1% volume is injurious to the product quality, while smaller quantities of benzene can be tolerated, that is less than about 0.1% volume. To insure that the isomerization reaction will not be retarded or to maintain long catalyst life, it is, necessary to remove aromatic hydrocarbons from the feed or to substantially reduce their concentration in the feed stream. Moreover, benzene cannot be removed from the isomerization feed by fractionation to the level required without losing much of the normal hexane and part of the methyl pentanes, which are desirable isomerization feed-components because of their low octane ratings, since the efiective boiling point of benzene will be about the same as that for normal hexane. The separation of aromatics has been done by such methods as solvent extractiomextractive' distillation, and low-pressure hydrogenation using a catalyst such as palladium. Operations of this type are costly, however, and the need for better methods for removing aromatics has continued to exist.

In accordance with the present invention, the removal of aromatics from the isomerization feedisaccomplished by .a continuous cyclic adsorption process'wherein a paraifinic feed stream is conta'ctedwith an adsorbent material, selective for aromatic hydrocarbons and wherein a novel and efiicient method of recovering the ad- 1 sorbed aromatics is employed and wherein all contacting is effected substantially in the liquid phase. More pan ticularly, the improved adsorption process of the invention employs a series of adsorbent zones (preferably six) with said zones operating on a four-step cycle with all afiins being displaced out of the zones, (B) heating one of the zones containing the adsorbed aromatics with a hot isomerizate stream containing limited amounts of aromatics, is effected in a third zone, with a further increment of parafiin material passing out of the zone, (C)

vapor-phase for the isomerization of normal parafiinsj to eluting of the aromatics from the adsorbent zone. which has been previously heated by the hot isomerizate/aromatic stream of step B with hot isomerizate, with isomerizate plus aromatics passing out of the zone, and (D) cooling the adsorbent zone with an aromatic-free treated parafiin fraction which has previously had the aromatics removed therefrom by means of step A of the instant process and displacing the isomerizate/aromatic material therefrom. It should be noted that by using hot isomerizate in elution step (C). aromatics with their higher octane quality are recovered in the isomerizate which is passed out of the system as product; and further that by recycling the hot isomerizate/aromatic material withdrawn from step D to step B a three-fold benefit is achieved, i.e., (l) iso/normal paraffin exchange, (2) heating of the zone, and (3) partial elution of the aromatics from that portion of the zone initially raised to the elution temperature. r

lt'has been found that by means of the instant invention the heretofore unattractive fixed-bed adsorption process is unexpectedly an etiicient means of reducing the benzene concentration in an isomerization feed to an acceptable concentration. Furthermore. by providing six adsorption zones, two zones are used in the adsorption and elution steps while one zone each is employed in the heating and cooling steps. When two zones are provided in the;adsorption and elution steps, respectively, it is preferred that these zones are charged with their respective feeds out of phase; that is, when one adsorption zone is for example approximately saturated with benzene, adsorption is just beginning in the second adsorption zone. A similar situation would preferably prevail in the two zones directed to elution, thereby assuring a continuous cyclic process.

It is well known that particulate solids, suchjas activated charcoal, clay, synthetic resins, silica gel and the like, when brought into contact with certain liquids or solutions selectively attract certain components from these liquids by a phenomenon known generally as adsorption. As used in this disclosure, the term adsorption has a broad meaning which is exemplified by the affinity of certain hydrocarbons such as aromatic materials for adsorbent materials such as silica gel and clay particles.

It is also well known that such granular solids termed adsorbents are selective in their affinity for certain components of liquid hydrocarbon mixtures and that this preference is determined in part by the quantity of the various components in the mixture available for adsorption. That is, if two materials are available to the adsorbent in equal amounts, one will be prefe. entially adsorbed, but if the latter were in a higher concentration, it could be selectively adsorbed in place of the former. The selectivity of an adsorbent for component A in a liquid mixture of components A and B can be measured by the following: r

QOIlCellliltItlQll of A (adsorbed) Concentration of A (in liquid) Qoncentration of B (adsorbed) Concentration of B fin liquid) Therefore, the total of either component to be adsorbed is roughly dependent on the total concentration of each Selectivity for A== in the solution.

3 accomplished through a mass action or concentration effect. Such substances are described as desorbents or eluants. Eluting of the selectively'adsorbed materials may also be effected by means of heating the adsorbent to a temperature at which the adsorbed material is no longer strongly attached to the adsorbent.

The usual procedure for carrying out selective adsorption operations of liquid feed mixtures by means of solid adsorbents is merely to filter the feed through a stationary body of adsorbent until the adsorbent becomes saturated with the selectively adsorbed component or until the effectiveness for making further separations of the components has decreased to an undesirable level. Introduction of the feed charge is then discontinued and the adsorbed material (adsorbate) is removed from the contact mass (adsorbent). This may be accomplished by washing the adsorbent with a suitable solvent, by heating, by blowing with a gas such as helium or flue gas, or by a combination of such methods. The adsorbent having the adsorbate removed may then be reused in the adsorption of another portion of feed in another cycle of operation.

The numerous advantages of the instant process over various conventional fixed bed adsorption systems and various liquid-liquid extraction processes and molecular sieve separations will become apparent from the following description of the invention.

It has been determined that the adsorption aflinity of certain adsorbents such as silica gel is'essentially the same for both isoand normal-parafiin materials and that the relative adsorption aflinity of these adsorbents for isoarid normal-paraflins is not significantly affected over the operating temperature range employed in the instant process. The temperature fronts in the heating and cooling steps are similar, but inverted. Accordingly normal-parafiins are used for paraffin exchange and as the adsorbent coolant in cooling step (D) at approximately 100 F.; and an isomerizate/benzene mixture is used in heating step (B). Similar, but inverted, paraffin exchange fronts result during these steps.

The drawing consists of FIGURES I and II. FIGURE I is a schematic diagram of a preferred embodiment of the invention and FIGURE II is a diagram showing the concentration of benzene in the desorption zone etfiuent as a function of time or desorption zone volume.

Referring now to FIGURE I of the drawing, six zones (1-6) containing 28-300 mesh silica gel adsorbent are employed in the following steps (A-D) of the continuous cyclic process of the invention:

Step A (benzene adsrpti0n).A paratfin feed containing benzene is introduced into adsorption zones 1 and 2 by'means of lines 8 and 9 respectively with a benzene free (less than about 0.1% by volume benzene) paraffin material being withdrawn overhead through lines 10 and 12, respectively. The benzene-free parafiin material passes out of the system to isomerization reaction zone (not shown) by means of line 14.

Step B (heating and iso/normal exchange and initial benzene desorpti0n).--An isomerizate/benzene stream at a temperature of about 350 F. is withdrawn from cooling zone 6 by means of line 16 and introduced into the bottom of heating zone 3 to effect heating of the zone and to. force out of the zone the paraflin material contained in the adsorption zone during step A. Initial benzene desorption also is effected in a portion of the zone. A further amount of benzene-free parafiin is displaced out of zone 3 through line 18 and joins the stream passing to the isomerization reaction zoneby means of line 14.

Step C (benzene desorptian).-Eluting zones 4 and are used to elute benzene from the isomerizate/benzene warmed zone from step B. Hot isomerizate from the reaction zone (not shown) having a temperature of about 350 F; is introduced into eluting zones 4 and S by means of lines 20 and 22, respectively, with the isomerizate/benzene stream passing overhead and out of the L system as the finished product thereof by means of lines 24 and 26 respectively.

Step D (cooling and normal/iso exchange).a stream of benzene free paraffins is withdrawn from line 14 by means of line 28 and introduced into cooling zone 6 as the cooling and normal/iso paraffin exchange material. The isomerizate/benzene stream displaced out of zone 6 through line 16 is introduced to heating zone 3, as described above. Thus, cooling zone 6, at the completion of step D, contains the adsorbent with a benzene free parafiin material at a temperature of about 100 F. and is ready for adsorption step A.

Referring once again to FIGURE I of the drawing for a more detailed description of the invention: six zones 1-6 are employed in a continuous cyclic adsorption process.

Step A.In the first step of the four step cycle, benzene is adsorbed from a paraffin feed fraction, containing benzene in concentrations from about 0.5 to about 6% volume, in two adsorption zones 1 and 2 at about 100 F. As discussed previously, these adsorption zones which adsorb the benzene from the paraffinic feed are out of phase from each other. That is, when zone 2 is approximately 50% saturated with benzene, adsorption is just beginning in zone 1. In the benzene adsorption step the paraffin fraction containing benzene is introduced into the bottom of the adsorption 'zones by means of lines 8 and 9 respectively. The introduction of the benzene containing feed into adsorption zone 2 is terminated before the zone is completely charged with the paraffin/ benzene feed. That is, a portion of the zone remains wherein the adsorbent has not selectively adsorbed aromatics thereon.

. The area of zone 2 above the dotted line represents the adsorbent free from benzene A paraffin stream essentially free from benzene (containing less than 0.1% by volume benzene) passes overhead, out of adsorption zones 1 and 2 by means of lines 10 and 12, respectively and into line 14 and passes ultimately by means of line 14 into an isomerization reaction zone (not shown).

Step B.-The partially charged adsorbent zone from step A containing paraflins and benzene is then heated to temperatures approaching 350 F. by introducing a hot isomerizate feed containing limited amounts of benzene into heating zone 3. (The source of this isomerizate/ benzene stream will be discussed below.) Therefore, when adsorption of benzene ceases in adsorption zone 2, this zone advances to become heating zone. 3 wherein the heating process of step B takes place. That is, a hot isomerizate stream containing some benzene withdrawn from cooling zone 6 is introduced into the bottom of heating zone 3, which contains the paraflin/benzene charged adsorbent from step A, by means of line 16. The normal parafiins essentially free from benzene is passed out of zone 3 by means of line 18 and are subsequently passed to the isomerization reaction zone by means of line 14. During step B the normal parafiins adsorbed by the silica gel adsorbent during step A are exchanged for the isoparafiins contained in heating stream 16. Moreover, the paraffin materials making up a part of the interstitial fluid in zone 3 are similarly displaced by the incoming hot isomerizate stream containing limited amounts of benzene. Therefore, the overhead produce withdrawn from zone 3 in step B consists essentially of the parafiin material free from benzene and along with the overhead product from adsorption step A can be introduced directly into an isomerization reaction zone. By this approach a relatively sharp iso/normal paraffin exchange front passes through the zone, and this front is unaffected by the temperature front which follows it through said zone. Thereby, at least the lower portion of the zone is raised to the elution temperature resulting in the initial elution of benzene. If the iso-normal paraftinic exchange which occurs in step B were excluded and adsorption zones 1 and 2 were eluted directly, the overhead therefrom would consist essentially of the benzene which had been adsorbed from the .feed and the normal parafiins adsorbed on the adsorbent, the latter also comprisev a major portion of the adsorbent interstitial material.

Therefore, the benzene/paratfin effluent from such a direct eluting process would obviously be unsatisfactory 1 as isomerization feed and would therefore have to be by-passed around the reaction zoneresulting in a' lower octane product, as the low octane parafiin material would not have been isomerized.

Step B also accomplishes an additional adsorption step in addition to the iso-normal exchange. That is, that portion of the adsorption zone from step A which did not have benzene adsorbed thereon can now be used to adsorb benzene contained in the interstitial liquid in the zone at the beginning of step B. Further, this unsaturated portion of the zone serves to adsorb any minor amounts of benzene contained in the isomerizate/ benzene feed which, is introduced into .the bottom of the adsorption zone, and to adsorb benzene which may be eluted from the lower portion of the zone during the initial heating/ eluting process, so as to achieve a separation of the iso/ normal exchange front from the benzene desorption front. A unique advantage of the instant process is that stream 16 can contain benzene and that the benzene concentration in the isomerizate/benzene stream (16)v can be at. least as high as the benzene concentration in the feed stream (9). However, the efficiency of the process is improved when the benzene concentration is somewhat less than the concentration in the feed, i.e., that concentrfation represented by the tall portion of FIGURE II (to discussed below). i l t As discussed previously, it has been found that the adsorption affinity of silica gelis essentially the same for both isoand normal paraffins and this relative adsorption affinity is not significantly aifectedover the operating temperature range. of the process. The temperature front passing through heating zone 3 during step B is similar but inverted to that in cooling step D (to be discussed below). Therefore, the iso/normal paraffin exchange front occurring in steps B and D should be similar but inverted for iso exchanging normal and normal exchanging iso-paraiiins from the interstitial and pore volumes of the adsorbent. The sharpness of these fronts during the iso/normal parafiin exchange steps affects the efiiciency of this process, since it is a measure of the iso-normal parafiin mixing that occurs during each cycle of operation. That is, if the exchange front is not sharp and clearly defined, the amount of normal paraffins by-passing the reactor or conversely the amount of isomerizate being recycled to the reactor would render the instant process less attractive. However, it has been found that these exchange fronts are uniquely sharp and distinct thereby minimizing iso-normal paraflin mixing and reducing bypassing and recycling to a satisfactory level in the process. Although the sharpness of the iso/normal paraffin exchange front affects the efficiency of the instant process,

some mixing will occur due to the non-piston-like flow ofthe fluids through the zones. amount of mixing overany period of time can be adjusted within limits by increasingthe cycle time or by increasing .material is passed overhead by line 18 with the isomerizate displacing the normal parafiin not only as the adsorbed material on the adsorbent but also as the interstitial material thereof. Further, a portion of the benzene contained in the isomerizate wash stream may be adsorbed in that section of zone 3, which has not been saturated with However, the absolute I 6 benzene. The benzene contained in the interstitial material present at the beginning of this step and some benzene which may be eluted during heating is adsorbed by the free adsorption section of the zone. The iso/normal exchange front advances through zone 3 with a temperature front following behind said front. That is, the isomerizate containing benzene has temperatures approaching 350 F.

.and is therefore also heating adsorption zone 3 in step B.

At a point prior to where the effluent from zone 3' contains any benzene or isomerizate/benzene, zone 3 is switched to step C, wherein essentially complete benzene desorption (elution) takes place. It is understood, of course, that the temperature front in zone B probably will not have reached the upper end of the zone. Therefore, this temperature front from step B continues to advance toward the top of the zone in eluting step C. It is obvious that elution of benzene is effected most efficiently in that part of the zone where the temperature has been increased.

Step B of the process is quite complex as three different fronts are moving through this zone during this step of the cycle. That is, there is a temperature front, an isomerizate/normal paraflin exchange front, and a benzene desorption front passing through the zone. The relative position of these fronts is important. The sharpness of the iso/norm-al paraffin exchangefront is essentially independent of temperature (as discussed above) whereas the diffuse benzene desorption front is strongly dependent upon temperature (as discussed above). step B is not started before complete saturation of adsorption zones 1 and 2, some provision must be made to insure a separation of the benzene and the iso/normal paraffin exchange fronts. For example, the addition of a small heat exchanger can be used to provide an initial pulse of cold recycle isomerizate. That is, cooling an increment of the isomerizate can result in a sufficient separation (by delay) between the iso/normal paraifin exchange front and the benzene desorption fronts.

Step C.--The two adsorption zones used to adsorb benzene in step A (now defined as eluting zones 4 and 5) are desorbed by a hot isomerizate stream withdrawn from the reaction zone (not shown). The isomerizate stream introduced into zones 4 and 5 by means of lines 20 and 22 respectively has a temperature of about 350 F. and is introduced into the bottom of said zones. Elution zones 4 and 5 contain the adsorbent with benzene and isomerizate adsorbed thereon and also contain isomerizate and some benzene as interstitial fluid. Introduction of the hot isomerizate results in eluting the benzene from the adsorbent. The eluted benzene passes out of the zone with the isomerizate as finished product by means of lines 24 and 26 respectively.

However, although benzene is strongly adsorbed on the silica gel resulting in a sharp adsorption front in adsorption step A, desorption of the benzene is effected with a less strongly adsorbed material, that is, hot isomerizate, with a dilfuse front resulting during elution step C of the process. Therefore, not all the adsorbed benzene is readily eluted by the hot isomerizate in step C of the process.

A more detailed discussion of the diffuse benzene front occurring in desorption step C is warranted. FIGURE 11 of the drawing is a schematic diagram showing the concentration of benzene in the desorption zone effluent as a function of time or zone volume during the desorption process. It will be noted that the concentration of eluted benzene in the isomerizate rises to a value approaching the original equilibrium value (that is, the concentration of 'benzene in the interstitial liquid at desorption temperature corresponding to that concentration of benzene originally present in the pore liquid at the completion of the adsorption step) and then decreases. The benzene concentration in the tail portion of the curve is actually quite low compared to the original equilibrium value.

Therefore, if

' This isomerizate/benzene efiluent obtained from step D has the fol-lowing advantages when used in step B:

" sentially complete elution is directly dependent upon the .volume ratio of eluantzfeed.

It is understood therefore, that by using the herein-described recycle stream of isomerizate/benzene, a more satisfactory ratio is achieved. Alternatively, lower eluting temperatures can be employed when this recycle stream is used. Quite uniquely, the overhead product from step C which is the finished product of the process contains the contaminant which was removed initially'from the feed mixture. That is, the benzene contaminating the isomerization feed is now taken overhead in desorption step C with the isomerizate, as the end product of the process.

Step D.The benzene-free parafiin stream withdrawn overhead from steps A and B (or a portion thereof) is introduced into the bottom of cooling zone 6 by means of line 28 wherein said zone a normal paraffin/isoparafiin 1 exchange occurs with isomerizate plus a small amount of a benzene (the tail portion of FIGURE 11 discussed above) I being displaced out of the zone and passing overhead by means of line 16 as the heating medium for step B. This stream is introduced into the bottom of heating zone 3.

J Zone 6 is thereby cooled to approximately 100 F. "aha contains the adsorbent with the normal paraffin adsorbed thereon and the normal paraffin also serves as the interstitialmaterial of the adsorbent. The zone thus treated is ready for adsorption of benzene from a .paraffin feed by means of Step A. It is obvious that the temperature front in zone 6 will lag behind the norm'al/iso parafiin exchange front and further it is not necessary that this front advance to the top of zone 6 during step D.

Another novel feature of the instant invention is that only two streams, i.e., feed and isomerizate, plus a series of temperature changes are involved in the instant process, whereas most adsorption processes employ a third stripping material which is separated from the other materials by a conventional separation process, such as distillation, or depend upon various pressure changes so that vaporization plays a significant role in the removal of one or both of the two liquids from the adsorption step.

It has also been determined that at lower benzene concentrations, shorter beds or longer cycle times can be used. Further, the elution temperature requirements for the removal of benzene from the beds for varying volume ratios of eluent to fresh feed have been determined. The elution temperature of step C of the process could be significantly lower, particularly in view of the potential advantages of the isomerizate recycle concept disclosed in step B, as discussed above.

I In the instant adsorption process there are certain limits to the percolation rate which can be used. By percolation rate is meant the volume flow rate per unit cross-sectional area of adsorbent. A practical limit to high flow rates is the resultant pressure drop. Flow rates from about 0.5 to about 6.1 g.p.m./sq. ft. are most attractive.

In a preferred embodiment of the invention, the isomerizate used in step C is generally withdrawn from an HCl stripper provided in the isomerization process. This isomerizate is first passed through guard beds, which are included for the removal of catalyst components such as HCl and other possible adsorbent contaminants.

An adsorbent which will preferentially adsorb aromatic components from the particular isomerization feed mixture employed must be used in conducting the instant separation process. Preferably an adsorbent which has a high adsorption capacity and a high degree of selectivity for aromatics between aromatics and the paraffinic components of the feed mixture is employed. Among the commercially available adsorbents, silica gel and activated carbon have adsorptive properties which are especially suited for use in this type of separation. Silica gel is especially effective for selectively adsorbing benzene. It is to be understood, however, that the process according to the invention may be practiced with other types'of adsorbents and in fact with any adsorbent which exhibits a substantial selectivity between aromatics and the paraffinic components of the particular feed mixture to be treated.

It is understood that streams or flows and the like to adsorbent zones in the process could be introduced into the various zones thereof at the top rather than at the bottom as disclosed in FIGURE I. In fact under certain conditions adsorbent attrition lifting" and other adsorbent problems can be alleviated by introducing the feed stream at the top of the-zone and withdrawing the eflluent at the bottom.

The particle size and physical shape of the adsorbents used, assuming a porous structure, varies over a-wide range of mesh sizes. .For example, mesh sizes between about 20 and 300 have been found convenient. In a preferred'embodiment, mesh sizes between about 28 and 200 are employed. Granular particles are particularly preferred in this operation. However, pellets or specially formed adsorbents can be employed. Moreover, another advantage of the instant invention is that the attrition of the solid adsorbent is maintained at an economically attractive rate.

The auxiliary apparatus employed in this process may be any conventional or convenient type known to those skilled in the art. For simplicity, the drawing does not show all the pumps, tanks, heat exchangers, valves, bypasses, vents, reboilers, condensers, and other auxiliary equipment that may be necessary for the proper operation of this process, but the inclusion of which will be evident to those skilled in the art.

The operating conditions, i.e., temperature and pressure of the adsorption zones, may range within wide limits provided that all fluids within the zones are maintained substantially in the liquid phase. Suitable tem. peratures for the adsorption zone range from the melting point of the feed mixture up to about the boiling point of the mixture, of pressures from 0.1 p.s.i.g. to about 500 p.s.i.g. assuming the feed mixture is stable under these conditions. In a preferred embodiment of this invention, a pressure of about 300 p.s.i.g is maintained in the adsorption zones during the adsorption step of the process. In a further preferred embodiment of the invention, the adsorption zone is operated at a temperature of about F. Step B is preferably operated at a temperature approaching about 350 F. It is understood that zone 3 in step B is undergoing a temperature transition, i.e., from about 100 F. to about 350 F. The desorption step C is preferably operated at about 350 F. In the cooling step, the resultant cooled adsorbent containing pure paraffins has a temperature preferably of about 100 F.

Although a preferred embodiment of the invention employs six separate zones, it is understood that the invention should not be so limited. That is, the four steps of the cyclic process can be achieved in various combinations of zones such ranging from 4 to 24.

We claim as our invention:

1. A continuous cyclic adsorption process comprising adsorption in a plurality of sequentially operating zones each provided with an adsorbent selective for aromatics and wherein each zone, stepwise and in turn, passes through the following steps:

(A) A paratfin feed stream containing aromatics is introduced into the zone and a parafiin stream essentially free from aromatics is recovered therefrom,

(B) A stream comprising isomerizate and aromatics is introduced in an amount sufiicient to heat at least a portion of the zone and a further amount of parafiin essentially free from aromatics is recovered therefrom,

(C) A hot isomerizate stream is introduced into the at least partially heated zone in an amount suflicient to elute the selectively adsorbed aromatics and a mixture comprising isomerizate and aromatics is recovered from the zone as the product of the process, and

(D) A portion of the cold paraffin stream essentially free from aromatics recovered from step A is passed through the zone in an amount. sufiicient to cool at least a portion of the zone and a stream comprising isomerizate and aromatics is recovered from the at least partially cooled zone, and introduced as the heating material for step B.

2. A process according to claim 1 wherein step A is operated at about 100 F., step B is operated at about 350 F., step C is operated at about 350 F., and step D is operated at about 100 F.

3. A process according to claim 1 wherein the adsorbent is silica gel.

4. A process according to claim 1 wherein the aromatics consists essentially of benzene and the parafiln rnaterial consists essentially of an isomerization paraflin feed.

5. A process according to claim l wherein'sim zones 1 are used and wherein two zones which are out of phase with each other are employed in each of steps A and C.

6. Aprocess according to claim 1 wherein pressures and temperatures are maintained such that all fluids'are substantially in the liquid phase.

7. A continuous, cyclic, adsorption process carried out in the liquid phase comprising adsorption in a plurality of sequentially operating zones each provided with an adsorbent comprising silica gel and wherein each zone, step wise and in turn, passes through the following steps:

(A) Introducing an impure paraffin feed for isomerizavolume of benzene into the zone whereby a paraflin isomerization feed having less than about 0.1% volume of benzene is recovered, i

(B) Introducing an isomerizate stream recovered from step D containing benzene in an amount sufiicient to heat at least a portion of the zone whereby a further amount of paratfin essentially free from benzene is recovered,

(C) Introducing a hot isomerizate stream into the at least partially heated zone in an amount suflicient to elute the benzene whereby a mixture com prising isomerizate and benzene is recovered from the zone as a product of the process, and

(D) Passing a portion of the cold isomerization feed recovered from the step A through the zone in an amount sufiicient to cool at least a portion of the zone whereby a stream comprising isomerizate and benzene is recovered from the at least partially cooled zone and introduced as the heating material in step B in such a manner that iso-normal paraflin exchange occurs.

8. The process of claim 7 wherein step A is operated at about F., step B is operated at about 350 F., step C is operated at about 350 F., and step D is operated at about 100 F.

9. The process of claim 7 wherein the stream introduced into step B contains benzene in an amount less than the concentration in the initial feed.

References Cited by the Examiner UNITED STATES PATENTS 2,628,933 2/1953 Eagle et al. 260674 2,870,230 1/ 1959 Scott et al 260-674 2,937,215 5/1960 Bleich et al. 260683.73 2,996,558 8/1961 Feldbauer 260-676 DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, C. E. SPRESSER,

' Assistant Examiners. 

1. A CONTINUOUS CYCLIC ADSORPTION PROCESS COMPRISING ADSORPTION IN A PLURALITY OF SEQUENTIALLY OPERATING ZONES EACH PROVIDED WITH AN ADSORBENT SELECTIVE FOR AROMATIC AND WHEREIN EACH ZONE, STEPWISE AND IN TURN, PASSES THROUGH THE FOLLOWING STEPS: (A) A PARAFFIN FEED STEAM CONTAINING AROMATICS IS INTRODUCED INTO THE ZONE AND A PARAFFIN STEAM ESSENTIALLY FREE FROM AROMATICS IS RECOVERED THERE FROM, (B) A TEAM COMPRISING ISOMERIZATE AND AROMATICS IS INTRODUCED IN AN AMOUNT SUFFICIENT TO HEAT AT LEAST A PORTION OF THE ZONE AND A FURTHER AMOUNT OF PARAFFIN ESSENTIALLY FREE FROM AROMATICS IS RECOVERED THEREFROM, (C) A HOT ISOMERIZATE STEAM IS INTRODUCED INTO THE AT LEAST PARTIALLY HEATED ZONE IN AN AMOUNT SUFFICIENT TO ELUTE THE SELECTIVELY ADSORBED AROMATICS AND A MIXTURE COMPRISING ISOMERIZATE AND AROMATICS IS RECOVERED FROM THE ZONE AS THE PRODUCT OF THE PROCESS, AND (D) A PORTION OF THE COLD PARAFFIN STEAM ESSENTIALLY FREE FRON AROMATICS RECOVERED FROM STEP A IS PASSED THROUGH THE ZONE IN AN AMOUNT SUFFICIENT TO COOL AT LEAST A PORTION OF THE ZONE AND A STEAM COMPRISING ISOMERIZATE AND AROMATICS IS RECOVERED FROM THE AT LEAST PARTIALLY COOLED ZONE, AND INTRODUCED AS THE HEATING MATERIAL FOR STEP B. 